1. Field of the Invention
The present invention relates to a method and an apparatus for driving a laser diode, and more particularly to a method and an apparatus to compensate for temperature characteristics of a laser diode in an optical communication system.
2. Description of the Related Art
Efforts have been made to use light for communication ever since the first ruby laser was developed by Maiman. For example, Corning Glass Inc., launched development of an optical fiber having a transmission loss of 20 dB/km and succeeded in commercialization thereof in 1974.
The optical communication is a communication technology that transmits light through an optical fiber made of glass or plastic. In such optical communication schemes, data transmission is done by converting information to be transmitted from an electric signal into an optical signal, delivering the converted optical signal to a destination, and converting the optical signal into the original electric signal on a receiving side. A typical example of such an optical communication system is an optical transmission system using an optical fiber transmission path as a transmission media, a semiconductor laser or a light emitting diode as a light source, and an optical detector as a light receiver.
Hereinafter, a transceiver for optical signal transmission, which transmits optical signals in such an optical communication system, will be described in detail with reference to the accompanying drawings.
FIG. 1 illustrates a structure of a transceiver for optical signal transmission in a common optical communication system.
Referring to FIG. 1, the transceiver is divided into a receiver unit 110 for photoelectrically converting an optical signal into an electric signal, and a transmitter unit 120 for photoelectrically converting an electric signal into an optical signal.
The receiver unit 110 includes a photo diode 111 for receiving the optical signal, a preamplifier 112 for converting a current signal generated by the photo diode 111 into a voltage signal, and a limiting amplifier (limiter) 113 for limiting an output of the preamplifier 112 to a desired voltage level.
The transmitter unit 120 includes a laser driving circuit (hereinafter referred to as “LD driver”) 121 for supplying a current to a laser, a laser diode 122 for generating an optical signal proportionally to a current generated by the LD driver 121, an automatic power control circuit (hereinafter referred to as “APC circuit”) 123 for compensating for a power change of the laser diode 122, and an automatic modulation control circuit (hereinafter referred to as “AMC circuit”) 124 for compensating for a modulation current of the laser diode 122.
In general, the LD driver 121 can vary a bias current and a modulation current of a laser with a change of an output optical signal.
FIG. 2 is a circuit diagram illustrating an LD driver which varies a bias current and a modulation current with a change of an output optical signal in a transmitter unit of a common transceiver for optical signal transmission.
Referring to FIG. 2, a laser diode 11 and a monitor photodiode 21 are connected to an input stage of the LD driver 121, and a current/voltage converter 30 is connected to the monitor photodiode 21. A peak error detector 40 and a bottom error detector 50 are connected in parallel to the current/voltage converter 30, and a voltage generator 60 is connected between the peak error detector 40 and the bottom error detector 50. Also, a modulation current driver section 80 and a bias current driver 90 are connected to the laser diode 11.
The peak error detector 40 includes a peak holding circuit 41 and an Op Amp 42, and the bottom error detector 50 includes a bottom holding circuit 51 and an Op Amp 53.
The modulation current driver section 80 includes FETs 81, 82, to which an input voltage Vcc and an output of the laser diode 10 are inputted, respectively, and a variable current source 83.
The bias current driver section 90 includes a variable current source 91.
The LD driver operates as follows.
The laser diode 10 is driven by the modulation current driver section 80 and the bias current driver section 90 to emit a part of an optical signal into an optical fiber. The photodiode 20 also receives a part of the emitted signal. The photodiode 20 outputs a current Ipd proportional to the received light. The output current is converted into a voltage Vo by the current/voltage converter 30. The converted voltage signal is input to the peak error detector 40 and passes through the peak holding circuit 41 to output a voltage Vop as shown in FIG. 3. The peak error detector 40 outputs an inverted signal of Vop by the Op Amp 42. The converted voltage signal is input to the bottom error detector 50 and passes through the bottom holding circuit 51 to output a voltage Vob as shown in FIG. 3. The bottom error detector 50 outputs an inverted signal of Vob by the Op Amp 52.
When the intensity of the optical signal generated by the laser diode 11 varies, for example, when the intensity of the optical signal is lowered with the passage of time as shown in FIG. 3, the voltage Vo also varies. The output of the peak holding circuit 41 varies like Vop with a peak voltage of the Vo signal, and the output of the bottom holding circuit 51 varies like Vob with a bottom voltage of the Vo signal. Thus, if both the output signals are input to inverting input stages of the inverter Op Amps 42, 52, outputs of the Op Amps 42, 52 become higher than those in an equilibrium state because both the output signals has been already lowered. As a result of this, currents of the variable current source 83 of the modulation current driver section 80 and the variable current source 91 of the bias current driver section 90 increase. Consequently, since the currents of the two variable current sources, the lowered optical power can be raised. Even in the opposite case, the LD driver 120 similarly operates according to such a principle.
However, the LD driver cannot compensate for temperature tracking errors. The monitor photodiode used for the laser module generally undergoes a change in its output current with respect to temperature around the laser module. If a current of the photodiode is measured with respect to a specific magnitude of laser power while changing the temperature around the laser module, the measured current varies from temperature to temperature. Thus, if temperature-by-temperature tracking errors of the laser module occur when laser power is arbitrary, compensating circuits of the prior art cannot compensate well for the optical power.
Also, the LD driver is difficult to construct the peak and bottom holding circuits, and all of the two holding circuits require a reset circuit. When peak and bottom levels of the laser gradually varies in an increasing direction, voltage tracking is possible because a holding capacitor current inside of the holding circuit is charged, but when the increased peak and bottom levels gradually vary in a decreasing direction, tracking of the decreased peak and bottom levels are impossible because the charged current cannot be discharged by the laser diode.